This invention relates to a band-pass filter for the microwave or SHF band using resonators each composed of a microstrip line, and is particularly effective when applied to microwave radio equipment.
There is known a band-pass filter for, e.g., the SHF band, in which .lambda./2-length resonators (.lambda. is the line wavelength corresponding to the central frequency f.sub.0 of their passband) each composed of a microstrip line formed on the front surface of a dielectric substrate between an input and an output transmission line which are connected to an external circuit. A grounded conductor layer is formed on the back surface of the dielectric substrate. In such a conventional filter, the adjacent resonators are coupled such that they align in parallel over the length of .lambda./4 of each resonator. However, due to a spurious resonance mode this arrangement may suffer degradation of its inhibiting characteristics in the vicinity of the integral multiple frequencies of the central frequency, for example, the double frequency of the central frequency.
If such a band-pass filter as cannot effectively attenuate signals outside the required band is applied to a radio transceiver of the SHF band, a receiving sensitivity may be lowered and extraneous waves may be emitted. To avoid these problems, it has been necessary to use additional circuits, making the equipment large and costly.